The classical, somewhat simplistic, view of the lipid bilayer component of biological membranes is a flexible selectively permeable barrier that separates different cellular compartments. More recently it has been shown that the lipid bilayer is dynamic and actively involved in membrane protein localization and function. A typical membrane contains over 200 distinct types of lipids and changes in bilayer composition can regulate numerous biological functions.
The lipid bilayer is involved in regulating membrane protein function because the bilayer is coupled to its embedded membrane proteins by hydrophobic interactions; the hydrophobic core of the bilayer shields the hydrophobic surface of the membrane proteins and vice versa. The shielding of the hydrophobic surfaces leads to hydrophobic adaptation of the relatively soft bilayer to the more rigid protein. This adaptation incurs an energetic cost, meaning that the hydrophobic coupling between membrane proteins and the host bilayer causes the equilibrium distribution between different membrane protein conformational states to depend on the bilayer's ability to adapt to the transmembrane hydrophobic domains off these different states. Therefore, changes in bilayer composition and material properties can modify membrane protein function—and it is well-established that experimental maneuvers that change the lipid bilayer hydrophobic thickness or monolayer curvature, e.g. by altering acyl chain length or lipid head group size, cause changes in protein function.
The bilayer's collective properties are determined not only by the lipid composition but also by the proteins and amphipathic molecules (amphiphiles) imbedded in the hydrophobic continuum.
Amphiphiles can accumulate at the cell membranes' bilayer/solution interface and thereby change the bilayer properties. A large portion of commercially available drugs are amphiphiles, and it long has been known that many drugs are membrane modulators—meaning that they, in addition to binding to their cognate protein binding sites, adsorb to the bilayer/solution interface to alter bilayer properties. In fact, many drugs and other molecules that are used to manipulate biological functions modify bilayer collective properties at physiologically/pharmacologically relevant concentrations to such an extent as to alter membrane protein function (such as genistein, neurotoxins, capsaicin, butanedione monoxime, anti-fusion peptides, curcumin, and poly-unsaturated fatty acids).
This is important because many new drug leads are amphiphiles, and they seem to be increasingly hydrophobic in nature. That is, many current drug leads have physico-chemical properties that are likely to make them potent modifiers of bilayer collective properties. This means that these compounds are likely to alter the function of a wide variety of membrane proteins, in addition to their desired target(s).
Side effects caused by high doses of these drugs therefore can be due to their effect on the bilayer collective properties. This raises questions as to which drugs and at what concentrations will alter lipid bilayer properties sufficiently to promiscuously alter membrane protein function.
Current drug development relies heavily on high throughput screening for drug lead discovery. Large libraries of millions of small molecules are commonly screened for a desired function often resulting in thousands of potential leads. These molecules need to be further screened to verify their specific interaction with the drug target and their suitability as potential drugs.
There are a number of methods available to measure bilayer collective properties, including: small-angle x-ray scattering (for measuring changes in bilayer thickness and intrinsic lipid curvature), membrane inserted fluorescent probes (to measure changes in the order and dynamics of the bilayer hydrophobic core) and micropipette aspiration methods (for measuring changes in bilayer elasticity). These methods usually detect changes in just a specific subset of bilayer properties under rather non-physiological conditions, and it may be difficult to relate the measured changes to changes in membrane protein function.
A broadly applicable method for determining changes in membrane collective properties in physiological systems is the gramicidin A (gA) channel-based method, because the channels' structural simplicity minimizes the risk that the changes in channel function are due to direct channel-drug interactions. gA single-channel electrophysiology has been studied extensively to examine many different questions, specifically, to measure non-specific membrane effects of marketed and investigational drugs. Using the single-channel technique, changes in bilayer collective properties have been measured for a variety of lipid compositions and after the addition of different small molecules. When working at the single-channel level, it is possible to directly monitor different gA channel properties. But to calculate population averages, it is necessary to collect hundreds of stochastic single channel events for each condition. Additionally the gA signal channel events are tiny (conductance is in the pA range) requiring specialized electrophysiological equipment to achieve the necessary signal-to noise ratio. To successfully test just one compound and/or lipid composition may take a highly trained person days, if not weeks, of collecting/analyzing single channel events. Only a limited number of molecules therefore have been tested in detail with the gA channel method, because the assay requires measurement of large quantities of single channel events, requiring an extensive experimental effort to test a single condition.